Heat-sensitive recording paper can produce an image due to a physical or chemical change of substances caused by thermal energy. A number of different types of recording paper for forming images have been studied.
In recent years there has been development of facsimiles and printers. In particular there has been a notable development of such devices utilizing a combination of a heat-sensitive recording paper prepared by applying a coating solution for forming a heat-sensitive recording layer containing a colorless dye such as Crystal Violet lactone, etc. and a phenolic compound as described in Japanese Patent Publication No. 14039/70 (U.S. Pat. No. 3,539,375) and a thermal head has been widely adopted for these apparatus. Such a process is referred to as a thermographic process.
This thermographic process has many advantages in that the recording paper need not carry out development because of first order color formation, the recording apparatus can be simplified, the cost of the recording paper and the recording apparatus is low, and noise does not appear because of non-impact recording. Therefore, it has been popularly used in recent years.
However, when a coating solution for forming a heat-sensitive recording layer is applied to paper, fine parts having a diameter of 0.5 mm or less which are not coated with the coating solution are formed on the surface of coated paper. Consequently, when typing is carried out by a thermal head utilized in devices such as a facsimile, there is no color formation on such parts, resulting in obscure recording.